Fat suppression manipulates the magnetization of fat so that it has no signal (approximately) at the time of the acquisition (readout). Thereby, a fat-suppressed gray-scale MR image depicts fat as black allowing for better differentiation of fat from, for example, fluids which are depicted as bright. Two fat suppression methods currently exist, fat saturation and fat inversion.
A saturation pulse has the advantage that the magnetization after the pulse is not dependent on that before the pulse, because it destroys it completely. In other words, it creates a “clean slate” for magnetization and makes it independent of the magnetization history. Thus, it has the major advantage that it creates an immediate and consistent steady state after the pulse. It has the disadvantage that it provides less separation between species of different T1 values and therefore creates less T1 contrast than inversion pulses. Also, it cannot create negative magnetization. Magnetization after an inversion pulse depends on the magnetization before the pulse. Therefore, if playing a series of inversion pulses and if a T1 species is not given enough time to fully recover to equilibrium magnetization (TReff<5*T1 of the species), then magnetization after each pulse is different initially. Only when the magnetization undergoes the same recovery curve between every pair of consecutive IR pulses will the magnetization be identical after each IR pulse. This is known as steady state and requires time (a certain number of TReff periods) until it is established. If data is acquired before steady state is reached, inconsistent T1 contrast or ghosting results. The advantage of an inversion is that it can create negative magnetization. In the context of fat suppression, this property of an inversion allows creating zero fat magnetization at a time further away from the preparation than for fat saturation. Also, slightly negative fat magnetization can be created which can improve fat suppression in the presence of B0- and B1-inhomogeneity.
Fat saturation applies a 90° radio frequency pulse at the fat frequency and spoils the created transverse fat magnetization so that longitudinal and transverse fat magnetization are zero at the beginning of the readout. This module is played immediately before the data readout and is known as CHEmical Shift Selective (CHESS) method. Because fat T1 is short, fat magnetization recovers quickly after the application of the fat saturation.
One major disadvantage of fat saturation is that it is B1-sensitive because the RF pulse that it uses is typically not adiabatic. Therefore the actually applied flip angle varies by B1 and thus by location. The actual flip angle does not exactly equal the prescribed ideal flip angle of 90° at any given location in the image. More importantly, fat saturation is B0-sensitive, because the RF pulse is frequency selective. In practice, the static magnetic field B0 varies by location changing the local fat and water resonance frequencies. Thus, the RF pulse is not applied exactly on the fat frequency at any given location and has locally varying efficiency. This modulates the actually applied flip angle in addition to the B1 effects described above. As a result, fat magnetization is typically not homogeneously suppressed by fat saturation.
Additionally, fat saturation erases the fat magnetization at the beginning of the readout. Whereas this is the optimal point in time for turbo-spin echo readouts, it is not optimal for gradient echo and steady state free precession (SSFP) readouts with linear reordering. For the fat to be suppressed in these latter sequences, the fat magnetization must be zero when acquiring the contrast relevant portion of the raw data, which is typically at the center of the readout. As fat recovers rapidly and starts with zero magnetization at the beginning of the readout, it has typically recovered significantly at the readout center so that it is not dark enough in the image.
Fat-selective inversion typically uses a spectrally attenuated inversion recovery (SPAIR) adiabatic pulse that is followed by a spoiling gradient and a time delay. After the application of the SPAIR pulse fat undergoes T1-recovery. The time delay is calculated so that the fat magnetization is nulled (crosses the zero-signal line) or is slightly negative during data readout following the time delay. This typically results in lower fat signal and hence a better fat suppression than by fat saturation. Also, the method is B1-robust, because the SPAIR pulse is adiabatic.
However, like fat saturation, fat inversion has a disadvantage, its frequency-selectivity which makes the fat inversion efficiency B0-dependent. As consequence, the fat recovery starting point is B0-dependent and varies locally. Inhomogeneous fat suppression is the consequence. This can be partially mitigated by setting the time delay to such value that fat magnetization is close enough to zero everywhere, slightly positive at some locations and slightly negative at others. This inhomogeneity is exacerbated if TReff of the module is too short for a full fat recovery between SPAIR pulses. Now, the steady state also locally varies and it is even more difficult to find the time delay where all locations have about zero magnetization at the contrast relevant portion of the raw data.
Even more critical, a major problem of fat inversion in this case (TReff of the module is too short for a full fat recovery between SPAIR pulses) is that it takes multiple TReff until steady state and thus consistent fat suppression is established. Often the leading non-steady state magnetization is acquired causing for example poor fat suppression for the slices acquired with the initial echo trains. This phenomenon can be appreciated in FIG. 1. In this example fat magnetization is fully recovered (at M0) before application for the first fat-selective inversion (FSIR) pulse. It takes the fat magnetization until the fourth turbo spin echo (TSE) train to reach steady state (where ‘steady state’ means that fat magnetization undergoes the identical recovery curve in periodic manner).
In FIG. 1 only the recovery curves after the last two FSIR pulses are at steady state. Therefore fat is depicted with a different gray-level for each of the TSE trains except for the last two, where fat magnetization is depicted identically and black. A solution to this problem was designed as shown in FIG. 2. A so-called “tickle pulse” was introduced before the first FSIR pulse so that the magnetization before this first FSIR is the same as before all other FSIR pulses. In theory, this solves the problem illustrated in FIG. 1 because fat is perfectly nulled (black) immediately, including the first readout. In practice, however, fat suppression for the leading slices is improved, but the problem is not completely solved because the “tickle pulse” is not adiabatic and therefore has a B1-dependent flip angle. Therefore, the optimal flip angle is not applied everywhere and consequently fat suppression is not optimal in the leading TSE trains.